I would suggest taking the first half of the TransAm from Astoria Oregoon to Pueblo, Co. Fly into Portland or Seattle and return from Denver. Much, much better scenery and roads with lower temperatures, especially climbing out of Hell and the Snake Canyon, you'll then be at elevation and nice cool nights for the remainder
I'm crazy and would give second thought to Southern Tier in the summer. If I did, I'd start at 3 am and finish by Noon and would sleep in motels as often as possible but it would not be fun